PLATE XX. 



Orthoceras medullars, Hall. 



Page iVi. 



Fig. 1. A fragment of a large individual, preserving several of the septa and a portion of the 

 outer chamber, together with considerable of the shell, showing the character of 

 the exterior surface. 

 '" 2. A section of the same individual, showing its elliptical form and the position of siphuncle. 



Orthoceras niagarense. Hall. 

 Page 416. 



Fis. 3. A view of the specimen described, showing the oblique undulations and several of the 

 septa. 



Orthoceras annulatum, Sowbrby. 



Page 411. 



Fig. 4. A natural cast of the interior of a part of the outer chamber. The annulations are very 



distinct and sharp, while the parallel encircling strise are very obscure or scarcely 



distinguishable. The longitudinal ridges are unusually well preserved, and give a 



nodose character to the annulations. 



" 5. A fragment of the septate portion of a specimen of this species, showing the obscure 



impressions of longitudinal ridges with the annulations not strongly elevated. 

 " 6. A view of the upper extremity of fig. 5, showing the broadly elliptical form of the sec- 

 tion and the position of the siphuncle. 

 The New York specimens occur in soft calcareous shale, and often preserve the marks of obscure 

 longitudinal ridges, interrupting the parallel transverse strias, giving a very obscurely nodose 

 aspect to the surface. This is shown in Pal. N. Y., Vol. ii, plate 64, fig. 1 a. The same character 

 is more distinctly shown in JIurchison's beautiful figure of this species, which in all its features 

 corresponds with the better preserved specimens in the Niagara group. . Those from Illinois and 

 Wisconsin present a great variety in the degree of this marking, owing to the more or less 

 complete solution and removal of the shell and the nature of the enclosing material. In many 

 specimens there are no nodes preserved on the surface. The undulating transverse striae are 

 rarely well preserved, but they are quite distinct occasionally. Specimens also occur in which the 

 undulating strias and nodes are both well preserved. The specimen, fig. 4, is quite an extreme one. 



